1. Field of the Invention
The present invention relates to a method and apparatus for upgrading low rank coal, and more particularly to a method and apparatus for upgrading low rank coal which is suitable to consecutively upgrade low rank coal.
2. Description of the Prior Art
There is known a technique for heat-treating low rank coal such as sub-bituminous coal and lignite through a plurality of stages to consecutively upgrade low rank coal, as disclosed in U.S. Pat. No. 4,249,909 by way of example, in which low rank coal is supplied by a first stage apparatus from the outside of a system for drying, the dried low rank coal is fed through a rotary feeder from the first stage apparatus to a second stage apparatus where the fed coal is carbonized and at the same time the pores of coal particles are blocked with material such as tar, then the low rank coal thus treated is fed through a rotary feeder from the second stage apparatus to a final stage apparatus to be cooled therein, and thereafter the cooled coal is taken out from the final stage apparatus to the outside of the system.
However, because such a conventional technique has not considered the problem of sealing between the inside and the outside of system as well as between the successive stage apparatus in the system, the outside atmosphere may flow into the system or inner gases may mix with each other between the successive apparatus in the system depending on the operating pressures of the respective stage apparatus, with the result that the process of upgrading low rank coal would be adversely affected.